Thin film multilayers are layered structures composed of several different materials and are commonly prepared for specifically envisaged applications. X-ray diffraction is a nondestructive technique particularly suited for studying their structural properties. However, extracting structural parameters from X-ray diffraction, such as spacing between individual atomic planes, interlayer roughness or strain, requires modelling and fitting the X-ray diffraction spectra. Here, we present a general kinematical model for wide angle X-ray diffraction of thin films that includes both the average atomic structure of the layers and structural disorder, for fitting the measured X-ray diffraction spectra. This model allows the extraction of composition (layer thicknesses), intralayer disorder and interfacial strain at the atomic scale that is assumed to be cumulative throughout the multilayer. In addition to the kinematical model, we also used an optical model for small angle X-ray reflectometry that allows us to obtain the composition (layer thicknesses and electronic density) and interfacial roughness. Unlike simpler fits of X-ray diffractograms that use functions like Gaussian, Lorentzian, or pseudo-Voigt, this model allows a more complete and accurate determination of the structure parameters. By fitting the measured profiles, it is possible to quantitatively determine both lattice constants and disorder parameters of a wide variety of multilayers. The model was applied to the characterisation of La0.67Sr0.33MnO3\SrTiO3\Bi0.9La0.1FeO3 trilayer films as a function of the different relative layer compositions in these nanostructures.